This invention relates to flight control systems of the type wherein the amount of fuel supplied to an aircraft propulsion unit is controlled to maintain the aircraft at or near a desired (i.e., selected) airspeed. More specifically, this invention relates to aircraft automatic throttle control systems and methods for achieving improved operation when the aircraft is subjected to atmospheric disturbances, including those disturbances commonly denoted as turbulence and wind shear.
It is well-known in the art that an automatic throttle control system, the aircraft employing the system, the aircraft propulsion system and the throttle mechanism for controlling the propulsion system collectively form a closed-loop control system that is subject to a number of somewhat contradictory design objectives and constraints. The primary system objective is that aircraft airspeed must be maintained above a minimum value (based on aircraft stall speed and a suitable safety margin) under all possible flight conditions and regardless of all other performance requirements. A second objective that is often of significant importance is system accuracy, i.e., the system's ability to maintain a constant selected airspeed within a predetermined tolerance. This aspect of system performance also affects or even determines the minimum airspeed obtainable with a particular system in that the difference between the minimum airspeed that can be maintained by the system and aircraft stall speed (the safety margin) must be equal to or greater than the system accuracy (airspeed tolerance).
To achieve a high degree of system accuracy, especially in the presence of rather abrupt changes in airspeed, the throttle control system response must be both very rapid and very precise. In fact, because the aircraft and its propulsion system exhibit a relatively long response time (i.e., a substantial amount of time is required for a change in throttle setting to result in the associated change in thrust and airspeed), optimum performance in response to rather abrupt changes in airspeed generally requires that the portion of the system which detects changes in airspeed and generates the throttle command signal present as little additional time delay as possible. In terms of frequency response, this means that the portion of the system that generates the throttle command signal (referred to hereinafter as the throttle controller) exhibit a relatively high cutoff frequency.
Accurate and precise systematic operation not only requires rapid throttle action but also requires adequate system damping in order to ensure that a change in selected airspeed or a change in propulsive thrust results in smoothly varying throttle control that does not overshoot or hunt about the proper thrust setting. Considered in terms of the throttle controller's frequency response, adequate damping of the overall system generally requires a relatively low cutoff frequency (i.e., a long response time) which, to a certain degree, conflicts with the requirement for a high cutoff frequency (fast response time) that is generally imposed by the previously discussed considerations pertaining to system accuracy.
Structuring a throttle control system to provide rapid response to undesired changes in airspeed while simultaneously obtaining the desired or necessary system damping is further complicated by the fact that a change or perturbation in airspeed error can be induced by atmospheric conditions as well as by an intended change in the selected airspeed by changes in thrust and drag that result from operation of the various aircraft flight control systems. In this regard, even if the system throttle controller exhibits an extremely short response time, the previously mentioned relatively long response time of the aircraft and its propulsion system will not allow a significant change in airspeed within the time period associated with relatively short term (high frequency) atmospheric disturbance such as those presented during periods of atmospheric turbulence. Thus, unless such a system includes means for compensating for turbulence-induced airspeed errors, a substantial amount of ineffectual throttle activity can be experienced whenever the aircraft encounters turbulent conditions. Such throttle activity not only causes annoying variations in engine noise, but also increases fuel consumption and exerts unnecessary stress and wear on the aircraft propulsion system.
Although compensation for turbulence-induced airspeed errors can be at least conceivably obtained by further control of the throttle controller damping factor or perhaps by configuring the system so that the generated throttle command signal is independent of all atmospherically-induced speed variations, such techniques do not result in totally satisfactory operation. In particular, to maintain satisfactory accuracy under all flight conditions, the system must also respond to atmospherically induced airspeed errors of a longer duration (lower frequency) than those associated with atmospheric turbulence. For example, when an aircraft navigates within a region of wind shear and changes altitude, a rather substantial airspeed error can develop if the system is not sensitive to atmospherically-induced speed variations at those frequencies associated with the wind shear.
Various automatic throttle control arrangements have been proposed in an attempt to overcome the above-mentioned constraints or at least reduce the compromises in system operation that result therefrom to the lowest possible level. In this regard, the approach commonly taken is the use of a multichannel (multiloop) throttle control system that utilizes both airspeed error and one or more signals based on the inertial acceleration of the aircraft as the system control parameters. More specifically, such a throttle control system can be represented by a system model (e.g., a basic block diagram) that is mathematically characterized by the control law: .delta..sub.t K.sub.a V.sub.e +K.sub.b V, where .delta..sub.t is the throttle command signal, K.sub.a and K.sub.b are gain factors that may be constants or functions of frequency and other system parameters, V.sub.e denotes the current value of airspeed error and V is representative of the current value of inertial acceleration along the longitudinal axis of the aircraft (i.e., instantaneous inertial acceleration along the flight path).
As is evidenced by the arrangements described in U.S. Pat. No. 2,948,496 to Joline; U.S. Pat. No. 3,448,948 to Reerink; and U.S. Pat. Nos. 3,840,200, 3,892,374, and 3,955,071 to Lambregts, utilization of longitudinal inertial acceleration as a control parameter can be advantageous both from the standpoint of system damping requirements and from the standpoint of compensating the system for atmospheric turbulence to thereby at least partially eliminate undue throttle activity. In this regard, and with respect to overall system operation, the use of inertial acceleration to improve system damping characteristics corresponds to rate feedback in that airspeed is the controlled quantity and inertial acceleration (which is proportional to the derivative of the controlled quantity with respect to time) is utilized as a control parameter that alters the magnitude of the throttle control signal. Thus, when the system is causing the aircraft to accelerate toward a selected airspeed, the inertial acceleration term decreases the magnitude of the system action substantially in accordance with the rate at which airspeed error is decreasing. For example, if a system is configured such that a "speed high" condition produces a positive throttle component K.sub.a V.sub.e and if the gain factor K.sub.b is positive, deceleration of the aircraft will result in a decrease in throttle command signal, .delta..sub.t, as the aircraft responds to the speed high condition and decelerates. As is known in the art, this means that K.sub.a and K.sub.b can be selected so that an abrupt, undesired change in airspeed initially results in a rapid corrective change in airspeed with the throttle command signal decreasing thereafter as a result of both the diminished airspeed error and the corrective acceleration term. Thus, when properly configured, a system utilizing both airspeed error and inertial acceleration as control parameters will rapidly return to the selected airspeed or capture a newly selected airspeed with minimal overshoot and unnecessary throttle activity.
The use of inertial acceleration to provide turbulence compensation is based on a somewhat more subtle relationship between aircraft airspeed and inertial acceleration than the relationship which results in the above-discussed "inertial speed damping". In particular, an atmospherically-induced change in airspeed is accompanied by inertial acceleration that is opposite in polarity (sign) relative to inertial acceleration associated with a similar or identical airspeed disturbance that is induced by either a change in aircraft propulsive force or selected airspeed. For example, if aircraft thrust decreases, both airspeed and ground speed decrease and, utilizing normal signal convention, inertial acceleration is negative (deceleration of the aircraft). On the other hand, if airspeed decreases due to a reduction in head wind (or an increase in tail wind), and compensatory throttle action is not initiated, ground speed normally increases (due to reduced drag) and the associated inertial acceleration is positive. Corresponding acceleration relationships exist relative to increases in airspeed that are induced by atmospheric conditions and by either changes in propulsive force or selected airspeed, with each of the above-mentioned patents to Joline, Reerink and Lambregts utilizing such relationship to provide turbulence compensation within an automatic throttle control system. For example, the system disclosed in the patent to Joline includes a first control loop (channel) wherein a signal that is representative of airspeed error is low-pass filtered to substantially eliminate all airspeed deviations attributable to atmospheric disturbances, including both turbulence and wind shear. This channel, which is identified as the "airspeed loop", establishes the behavior of the system relative to low frequency disturbances and a second control loop or channel is configured to control the high frequency response of the system. In this respect, the second control loop is responsive to a signal representative of the longitudinal inertial acceleration and includes a low-pass filter network having a lower cutoff frequency than the filter network utilized in the airspeed loop. By judiciously selecting filter cutoff frequencies, at least a certain degree of turbulence compensation can be achieved.
The patent of Reerink discloses two multichannel arrangements for producing a turbulence compensated throttle command signal based on airspeed error and inertial accleration signals that are each characterized by the previously mentioned control law. In the arrangements disclosed by Reerink, various signal components proportional to the time rate of change in airspeed error and inertial acceleration are superimposed (algebraically combined) so that speed errors induced by atmospheric disturbances produce minimal control action.
Although the systems disclosed by the patents to Joline and Reerink may provide adequate turbulence compensation in the least demanding situations, neither of these arrangements include means for providing satisfactory operation when an airspeed error is induced by low frequency atmospheric disturbances such as wind shear. For example, the arrangments disclosed by Reerink do not appear to include any means capable of, in effect, distinguishing between wind shear and turbulence-induced airspeed disturbances. Further, maintaining a satisfactory degree of responsiveness to wind shear by controlling the cutoff frequencies and even the order of the filter functions employed in the type of arrangement disclosed in the patent to Joline would not appear practical, at least in those systems that impose relatively stringent performance requirements. For example, an attempt to increase the order of the filter functions would introduce additional signal delay that would further increase system response time and would also substantially increase the complexity of the system.
The automatic throttle control systems disclosed in the above mentioned patents to Lambregts are characterized by the previously mentioned control law wherein both airspeed error and inertial acceleration are utilized as control parameters with the disclosed arrangements providing a certain degree of responsiveness to wind shear-induced airspeed errors. More specifically, the arrangements disclosed by Lambregts include a "wind shear detector" which, in effect, determines the difference between the current value of inertial acceleration and derived acceleration (time rate of change in airspeed error) and sequentially processes a signal representative of this difference with first order low-pass filter and first order rate limited filtering. The resulting signal, which is indicative of that part of the airspeed error that is caused by wind shear, is then combined with signals proportional to the total airspeed error and inertial acceleration in a manner which provides a measure of corrective throttle action during encounters with wind shear while simultaneously eliminating ineffectual and unnecessary turbulence-induced throttle activity.
Although the arrangement and technique disclosed in the patents to Lambregt may be satisfactory with many types of aircraft and under a variety of operating conditions, further improvement in wind shear response is desirable, or in some situations, necessary. For example, extremely demanding requirements can be imposed in an automatic throttle control system that is operational during landing approach wherein approach is to be made at minimum airspeed to thereby minimize landing distance. The prior art automatic throttle control systems have not been entirely satisfactory in such situations, especially relative to approaches under conditions of decreasing wind shear that cause a decrease in airspeed. For example, even the most accurate prior art turbulence compensated automatic throttle control systems exhibit an airspeed loss of as much as approximately 4 knots when executing landings under conditions of moderate, decreasing wind shear. In addition, in one such system an attendant loss of approximately one degree in flight path angle was experienced (due to an increase in angle of attack of approximately the same magnitude). Further, although a prior art system may be capable of restoring the aircraft to the selected airspeed, an extremely long recovery time can be required. For example, with respect to a system that was utilized prior to the development of the hereinafter described invention, a period of approximately 28 seconds is required for the system to return the aircraft to the selected approach speed.
Since relevant state of the art systems can maintain airspeed within approximately .+-.1 knot, under all normally encountered conditions other than wind shear, the airspeed loss and recovery time experienced with prior art systems substantially detracts from overall system performance capabilities and the general desirability of the system. Additionally, although such prior art wind shear response requirements can be accommodated within the system safety margins and thus not present unnecessary risk to the aircraft, a pilot (or other responsible member of the flight crew) will often disengage an automatic throttle control system during such a speed loss and assume manual throttle control in order to absolutely ensure that airspeed will not drop below the value necessary to maintain lift. It will be recognized that even occasional lack of confidence and unnecessary return to manual throttle control at least partially defeats the purpose of an automatic throttle control system and that a low confidence level often means that the flight crew will simply not utilize a particular system unless they have no other choice.
Accordingly, it is an object of this invention to provide an aircraft speed controller wherein turbulence-induced excess throttle activity is minimized.
It is another object of this invention to provide an aircraft automatic throttle control system that accurately maintains a selected airspeed when the aircraft is subjected to environmental disturbances while also eliminating ineffectual throttle activity due to relatively high frequency components of such disturbances, including those generally identified as turbulence.
It is yet another object of this invention to provide a turbulence compensated automatic throttle control system which remains responsive to wind shear induced airspeed changes without significantly increasing the system delay time and thereby detracting from system performance while operating under normal atmospheric conditions.
It is still another object of this invention to provide a turbulence compensated wind shear responsive throttle control system suitable for use during landing and approach maneuvers.